#6 Coper NMB Cable Limitation

Why is #6 Copper NMB cable's ampacity restricted to the 60deg Celsius rating even though it's actual manufactured ratings are to 90deg Celsius? It's ratings match type SE cable's specs, and it manufactured and rated to 90deg Celsius. I could pull the THHN conductors out of the NMB cable, pull them into conduit and now, they are rated to 90deg Celsius. I have heard that it is the rating of the jacket, that limits it, however the NMB cable's 30mm PVC jacket is the same as the SE Cable's 30mm PVC jacket. I have yet, to find a valid reason to the limitation. Please provide some insight, I feel, this is a rather VALID concern for everyone involved, electricians, manufacturers, homeowners, business owners, suppliers. I also believe this limitation should be changed.

I believe you are comparing apples to oranges. While a single THHN conductor may have a 90°C rating, derating must be applied when it's ran in various raceways or installations. I was always under the impression that the 60°C for nm-b was based on applying a derating to the nm-b since the number of conductors within a jacket is known.

Also while nm-b specifies the usage of THHN, for cost purposes the individual conductors are rarely marked as THHN, or marked at all.

I think he's talking about 334.80. I think the reason would have to do with the fact that it is widely used for residential and is usually located in attics which can get quite hot. I found an article which says that it's because of the limitations of the of the connections of the branch-circuit devices...

Yes! (my opinion - no reference to Code). The 90C or 60C is in reference to Max. Operating Temp. (of its environment) such as Heat Treat, Plenum or a Foundry.

Yes! (my opinion - no reference to Code). Solid Strand Conductors behave differently when connected to 'Devices' (such as those which Trip based on Thermal transfer to the sensing element) as opposed to the common Multi-Stranded Conductor.

Special Note: as Chris points out, if pulled from the Jacket, the Wire is (usually) not Labeled. The Code [NFPA-79 (2018) 12.1.1 & 12.4.2] requires that Wire must be Labeled with Wire Type (Insulation), Voltage Rating & AWG. Therefore, NEVER pull 6Ft. of Wire out from its Jacket of Cable Tray Multi-Conductor Cable supplying a Disconnect for the Sealtight jaunt to the Motor - as often done by most homeowners, business owners and electricians).

You can use the higher temperature rating but need to keep in mind the termination and UL label. If the cable assembly is UL labeled you will nullify the NMB label by separating the conductors from its sheathing. In addition very few terminations are designed for 90deg C wire. It is not permissible to connect a 90deg C rated wire to terminals with a rating less than the conductor. Your are going to find it difficult to find terminations rated at 90deg C. Most all circuit protectives are rated at 75Deg C or less. The other issue is the THHN 90Deg C wire itself. THHN wire must be run in a dry locations only and cannot be run underground or outdoors.

"It is not permissible to connect a 90deg C rated wire to terminals with a rating less than the conductor."

This statement is incorrect. One is limited by the weakest link in the chain. Hence, it is perfectly fine to connect a 90-degree C rated wire to a terminal with a 75-degree rating. One just has to use the 75-degree limitation for ampacity. You sure are confusing people with your superfluous answers. Don't confuse the newbies, please.

No confusion intended. Yes, I did not explain myself and yes you are correct but not necessarily right.

The first issue is that no one should be using the code as his basis for design. There should be an engineering element to any designers decision that goes beyond the code. In every set of building specifications there needs to be rules by the engineer of record that govern installation. One of my rules is to mate the wire temperature to the termination temperature not the wire ampacity to the termination temperature rating. There are so many variables in electrical systems for building that each situation must be evaluated on its own merit and experience by the engineer. Yes we have to use Code as the minimum but our design needs to reflect a higher level of thinking that involves quality assurance and control.

Here is a quote right out of Square D's digest: "It is acceptable to use conductors with a higher insulation rating than what is marked on the breaker, but the ampacity must be based on the conductor temperature rating marked on the breaker. So, circuit breakers marked for use with 75 deg. C rated conductors may be used with conductors rated for 90 deg. C if the conductors are sized per their 75 deg. C ampacity."

The code is not a design standard. I understand what this forum is about but for those who wish to design electrical systems I will not just hang my hat on the code as though it were gospel. Designing electrical systems goes way beyond code.

My rule is mute? When does the manufacturer change the lugs before construction, after the AHJ red tags the installation or when the feeder fails from over temperature and burns down the building? The courts do not expect or require the installer to follow code only that they follow the engineers direction. The engineer in todays world is fully responsible for design and code compliance in a court of law. Yes manufacturers can modify anything to fit site requirements and get a UL label. Of course economics plays a roll.

I am not sure where you come up with this stuff. It's like this: NFPA creates the standard, called NFPA-70. The NEC is most definitely a standard and it's the minimum requirement for electrical installations.

As far as lugs - contractors can order equipment with 90-degree lugs, or even special order the lugs from the manufacturer (made to go with that specific equipment) to be installed in the field. It happens all the time. It's electrical contracting 101.

"The courts do not expect or require the installer to follow code only that they follow the engineers direction."

Sorry Nick but unless you understand engineering and the liability associated with it you wouldn't know or understand this aspect of building design. In very few instances does the court side with the engineer in a court battle or law suite. The engineer is considered the expert or the one having the education in a court of law not the builder. The engineer is fully responsible for what is being installed on a project he has designed not the contractor. The engineer could create a set of specifications a foot thick to cover all conditions that might come up on a project but at what cost?And materials can be changed in the field but who pays for it? As I said the installer cannot do anything of his own initiative without the engineers approval. That is construction 101.

Nonsense. I've served as expert witness in over a dozen law suits. And no- the engineer isn't considered the "expert." The "expert" is someone who has more knowledge than the average layperson (legal definition). So in reality the expert witness is the person that is hired by legal counsel, based on their qualifications. Period. Not all engineers are experts. This begs the question -

Please review the ethics of a professional engineer and his legal profile when it comes to construction law. Definition of an expert is "a person who has a comprehensive and authoritative knowledge of or skill in a particular area" When a person goes to college and gets a four year degree in electrical engineering and then becomes licensed four years later as a professional electrical engineer where does that put his or her skill level over the installer in the construction food chain? The guy in the field cannot do anything on his own without first getting the engineers approval, this is the law. The governmental authorities have essentially made the engineer today responsible for just about everything that happens on a construction site and the courts follow.

Only a dozen or so law suites and that makes you an expert on the subject? Having been an electrical design professional since 1970 I have seen this industries liability risk change dramatically for the engineering community over the years. At one time the engineer did his engineering and was responsible for his engineering and the contractor was responsible for installations and code. Now The engineer [ at least in Florida ] has to put everything on his design docs including code references and installation details as a requirement for permitting

The expert witness is the person that is hired by legal counsel, based on their qualifications. Period. They are qualified by the judge presiding over the case. Not all engineers are experts. This begs the question -

Nick If you are a PE you would know that a registered engineer has behind him four years of college and four years as an apprentice. That is a total of eight years before he or she can sit for the PE exam. I'm pretty sure that make them an expert in their discipline. Does that make them a know it all or a savant? Not even close. My discipline and education is electrical engineering majoring in AC/DC power systems [ a dying discipline ]. Electronics was not taught in college when I went to school. My expertise includes electrical systems for all occupancy types with emphasis on power generation, healthcare, schools and the hi-tech industries such as aircraft and semiconductor manufacturing. I'm well aware of the application of engineering fundamentals, how to apply electrical materials and where the code fits into the equation. I'm also versed in many other aspects of building design. I have been an electrical consultant to the building industry and a design professional since 1970. Does all this make me infallible? If so, I wouldn't need errors and omissions insurance. I can roll out all of my resume if you are not satisfied. Does that mean I have memorized all volumes of NFPA and other codes and standards? Absolutely not! But I do have a very good basis to work with and a large book of brains as a reference.About code and standards:

A code is a model, a set of rules that knowledgeable people recommend for others to follow and does not take the place of engineering. Code is not a law until it is adopted into law. Code is the basis by which plans get reviewed under governmental authorities. A standard is a more detailed elaboration than a code. A standard may be defined as level of quality, achievement that is considered acceptable or desirable.[ Engineering ethics and liability rules ] A standard is a document that provides requirements, specifications, guidelines or characteristics that can be used consistently to ensure that materials, products, processes and services are fit for their purpose.

No, you are incorrect again. One would have to be hired by an attorney and then qualified by the judge presiding over the case. That is what an "expert" witness means. Merely being a PE doesn't automatically qualify you as an expert. Pleeeease don't make me repeat the context of previous posts. See Black's Law dictionary if you want.

All I do here is to try and answer NEC related questions. I give the correct answer as it pertains to NEC. If you like to have lengthy, expository, opinionated discussions about the engineering aspects of electrical energy, then you would be better suited to post your comments in an engineering forum. That would be my suggestion for you.

You have been incorrect in several posts, and on several different topics. If you (or anyone else) posts incorrect information, I try to correct it so as not to confuse others who may read the posts here, especially the newcomers and those starting out in the electrical trade. When I'm wrong, I admit it. When I don't understand something, I ask. But I never, never pretend to know something that I don't.

We are talking about Code and its application are we not? This is a forum about code and its application and the engineer is considered the expert in the building codes, the interpretation of the code and its application. I would say the design professional has something to say on this forum. My aim is to provide my 2cents in the conversation from a practicing engineers point of view and as I said code does not take the place of good engineering.Unless you have been involved with building designs, construction law and legal issues you wouldn't know what goes on at the top. Contractors, who are not licensed design professionals, cannot be held responsible for any part of the code that requires engineering expertise and training. The catch is that governmental agencies now require the engineer of record to assume full responsibility for the building design not the installer and the courts usually follow. The installer cannot do a single thing on a construction site without the engineer of record approval. Engineers must perform under a standard of professional behavior that requires adherence to the highest principles of ethical conduct. Engineers, in the fulfillment of their professional duties hold paramount the safety, health, and welfare of the public. The safety, health, and welfare of the public starts with the building code but does not end there.

Whether you like it or not building codes and its application is very much about engineering and a goodly amount of what is printed in NEC requires engineering expertise.

NEC article 90.1 Purpose. "The purpose of this code is the practical safeguarding of persons and property from hazards arising from the use of electricity. This code is not intended as a design specification or an instruction manual for untrained persons" Yes NFPA is referred to as "Codes, standards and recommendations" but feel the terms are used rather loosely. Compliance with NFPA standards is voluntary unless governmental agencies incorporate the wording into their regulations. The definition of code and standards are two very different issues when it comes to building design and engineering.

If you are replying to me, I'm not the moderator. I'm not "holding up" any responses. If people don't respond to you, maybe your question has already been answered. Secondly, I was being facetious in my previous question to Ron. Take a deep breath and relax.

It's not the same jacket. Off the top of my head, SE cable has a sunlight resistant jacket, whereas NM cable does not. So it's two different animals at least from the standpoint of the jacket. Does that help?

I can't provide the answer that you want to hear. SE and NM are two different cable types. SE is also "wrapped" with the bare conductor, and NM isn't. The cable manufacturers will also tell you that these are two distinctly different cable types. The NEC also has a different Article for each. They are different. You can't get there from here...

Post exactly what the inspector wrote on his correction tag/form and if there is a way to help you, I will try.

If a person takes the time to read 334.80, there are subtle hints as to the reasons NM cannot be used at more than the 60-degree rating.

Should one wish to change the requirements of 334.80, I'm sure that NFPA would look forward to your public input! The 2020 code is almost completed, and the next cycle to change codes would be the 2023, which is not yet open for public input.